This invention relates to a method for manufacturing a high-strength bolt mainly for an automobile. More particularly, the present invention relates to an useful method for manufacturing a high-strength bolt having excellent delayed fracture resistance and stress relaxation resistance in addition to a tensile strength (strength) of 1200 N/mm2 or more.
As a steel for a general high-strength bolt, used has been medium carbon alloy steel (SCM435, SCM440, SCr440 etc.) having a required strength by quench hardening and tempering thereof. However, in case that an increased tensile strength of beyond 1200 N/mm2 is applied to such a general high-strength bolt for automobiles and various industrial equipment, it is likely to cause a delayed fracture within the high-strength bolt. For this reason, the applicable condition of the high-strength bolt has been limited.
The delayed fracture is classified into two types, one generated in a non-corrosive environment and the other generated in a corrosive environment. It has been said that a variety of factors are intricately intertwined to cause the delayed fracture, and therefore it is difficult to identify the main factor. As the control factors to suppress the delayed fracture, known have been a tempering temperature, a steel microstructure, a steel hardness, a crystal grain size of the steel, contents of various ally elements and the like.
However, an effective method for suppressing the delayed fracture has not been established. Various methods have been proposed, but they are only in a process of trial and error.
Techniques for improving the delayed fracture resistance have been disclosed by Japanese Unexamined Patent Publication Nos. 60-114551, 2-267243, 3-243745 and the like. In these techniques, by adjusting contents of various main alloy elements, obtained can be a steel material for high-strength bolt having an excellent delayed fracture resistance regardless of its high tensile strength of 1400 N/mm2 or more. These techniques, however, cannot completely get rid of the possibility of generating such a delayed fracture. Therefore, the high-strength bolt obtained from the above-mentioned steel material has an extremely limited applicability.
On the other hand, a fastening bolt for use at high temperatures (including the above-mentioned high-strength bolt) has another problem that its proof stress ratio decreases when the bolt is in use, resulting in a phenomenon of lowering a fastening strength thereof. This phenomenon is called a relaxation (stress relaxation). In particular, when a bainitic steel, a pearlitic steel or the like rather than a hardened and tempered steel is used for the bolt, the resultant bolt may have a poor resistance to such a phenomenon (i.e., poor stress relaxation resistance). This phenomenon possibly causes an elongation of the bolt, which prevents the bolt from keeping the initial fastening strength. Therefore, for example when the bolt is for a purpose associated with an automobile engine, the bolt needs to exhibit a satisfactorily high relaxation resistance property. However, conventionally, the relaxation resistance property of high-strength bolts has been left out of consideration.
An object of the present invention is to improve the above-mentioned problems, thereby to provide a useful method for manufacturing the high-strength bolt having an excellent delayed fracture resistance and stress relaxation resistance as well as a satisfactory-level tensile strength of 1200 N/mm2 or more.
It is an object of the present invention to provide a method for producing a high-strength bolt having excellent delayed fracture resistance and stress relaxation resistance. The method includes steps of: preparing a steel material; drawing the steel material severely to obtain a steel wire; forming the steel wire into a bolt shape through a cold heading; and subjecting the shaped steel bolt to a blueing treatment at a temperature within a range of 100 to 400xc2x0 C. The steel material includes C: 0.50 to 1.0% by mass (hereinafter, referred to simply as xe2x80x9c%xe2x80x9d), Si: 0.5% or less (not including 0%), Mn: 0.2 to 1%, P: 0.03% or less (including 0%) and S: 0.03% or less (including 0%). And it has pro-eutectoid ferrite, pro-eutectoid cementite, bainite and martensite structures. The total area rate of them is less than 20%. It also has a pearlite structure as the balance. By this method, produced can be a high-strength bolt having excellent delayed fracture resistance and stress relaxation resistance in addition to a tensile strength of 1200 N/mm2 or higher.
The steel material used in the method, if necessary, further includes (a) Cr: 0.5% or less (not including 0%) and/or Co: 0.5% or less (not including 0%), (b) one or more selected from a group consisting of Mo, V and Nb, whose total content is 0.3% or less (not including 0%), and/or the like.